While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF, ArF or F2 laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 μm or less. These lithography processes use resist materials comprising various alkali-soluble resins as the base resin.
For resist materials for use in the KrF laser lithography, in fact, polyhydroxystyrene resins become the standard. For ArF resist materials, studies are made on poly(meth)acrylate resins utilizing carboxyl groups as the alkali soluble group and polymers of cycloaliphatic olefins such as norbornene. Of these, poly(meth)acrylate resins are practically used because of ease of polymerization.
As for the poly(meth)acrylate, JP-A 9-90637 proposes a combination of methyladamantyl(meth)acrylate as acid labile group-containing recurring units (or acid labile units) with lactone ring-containing (meth)acrylate as adhesive group-containing recurring units. JP-A 9-73173 discloses an acid labile group having a tertiary alkyl radical of alkylcycloalkyl form. JP-A 2000-327633 discloses an acid labile group having an exo-isomer of a tertiary alkyl group of alkylbicyclo[2.2.1]heptanol form. This acid labile group is easily acid eliminatable, requires only low activation energy for acid elimination, and provides for a high resolution and less PEB dependence. As the adhesive groups with enhanced etching resistance, JP-A 2000-26446 and JP-A 2000-159758 propose (meth)acrylate units having norbornane-lactone and oxanorbornane-lactone, respectively. These works resulted in ArF resist materials having significantly improved resolution. Also, JP-A 2003-113213 discloses a ternary polymer comprising acid labile units, lactone units, and hydroxyl-containing recurring units as derived from 3-hydroxy-1-adamantyl(meth)acrylate, for controlling the diffusion within a resist film of the acid generated by the photoacid generator upon exposure. This polymer is also used as a base resin in resist materials.
With respect to acid-catalyzed elimination reaction within the resist film, if acid labile units have high elimination reactivity, elimination reaction can take place even at low temperature. This allows for a lower PEB temperature, restraining thermal diffusion of acid during the PEB process. If the acid labile units are endowed with an appropriate fat solubility, the dose-dependent contrast of dissolution rate in developer may be increased. The low acid diffusion and high contrast contribute to an improvement in resolution, that is, enhancements of various process margins such as exposure latitude (EL), depth of focus (DOF), and mask fidelity in forming a fine size pattern. They are also effective for minimizing the pattern roughness (i.e., line edge roughness (LER) or line width roughness (LWR) in the case of line-and-space patterns or circularity in the case of hole patterns). Acid labile units having high elimination reactivity are expected to reduce the PEB temperature dependency of pattern size and to minimize dimensional variations caused by a slight difference of thermal history between different positions on the wafer surface. For these reasons, there is a need for acid labile units having an appropriate fat solubility as well as high elimination reactivity.